The week of 14-20 November saw extensive seismic activity around the world, including a tremor of magnitude 7.8 (M7.8) in the South Atlantic. Although the total number of earthquakes recorded on the United States Geological Survey’s real time earthquake map (including those of all magnitudes in the US and its territories and at least M4.0 elsewhere) was broadly typical at 1470, there was an unusually high number of tremors greater than M4.0 and M5.0.
The Week’s Largest Earthquake: M7.8 Scotia Sea
The week’s largest earthquake accounted for a significant proportion of the elevated number of recorded seismic events. At M7.8, the Scotia Sea tremor which occurred in the South Atlantic on 17 November is one of the largest of the year. It was preceded by a number of foreshocks, one of which registered M6.8, and a series of aftershocks. In total, the earthquake sequence included 47 events of ≥M4.5 at the time of writing.
Tectonically, the earthquake series is related to the boundary between the Antarctic plate and the much smaller Scotia plate, to the south of the Falkland Islands. The former is moving eastwards relative to the latter and the series of earthquakes propagated broadly in this direction over time. Despite its magnitude, however, the remote location meant that no damage occurred in inhabited areas and the fact that the movement was lateral meant that no significant tsunami occurred (a large tsunami requires significant vertical displacement of water.)
European Earthquakes: M4.9, Croatia
Although large earthquakes of the type experienced in the Scotia Sea are extremely rare in Europe (though not unprecedented: the magnitude of the 1755 Lisbon earthquake is estimated at around M8) the Mediterranean region is subject to regular smaller tremors. The convergence of Africa and Eurasia has led to a complex pattern of tectonic activity within the Mediterranean basin, resulting from both extensional and compressional activity.
Most of the earthquake activity in this region is centred on Italy and the subduction zone south of Greece and Turkey. This week, a tremor of M4.9 in Croatia proved to be the largest in the region. Although not associated with either of the two main regions of tectonic activity, a glance at a map of past seismic events indicates that tremors of this nature are uncommon but not exceptional.
U.S. Earthquakes: Hawaii
While the worldwide pattern of tectonic activity in the world in general was high, that in the US was low, with no single earthquake larger than M4.0 in the lower States. This offers an opportunity to examine Hawaii – an area in the centre of the Pacific plate far from any tectonic margins. In such areas, the source of earthquakes is not plate movements but volcanoes. Magma and gases moving within the volcanoes cause minor – and sometimes major – tremors which provide volcanologists with clues to the dynamics of volcanoes and, in some cases, the potential for eruptive events.
Major Earthquakes, Minor Earthquakes
To be interesting, an earthquake does not have to be either large or devastating. Although the Scotia Sea tremor was one of the largest of the year so far, it had little impact. Meanwhile, the minor tremor in Croatia was unusual enough to cause comment, while the continued pattern of minor tremors in Hawaii are clues to a different source of seismic activity.
USGS. Real time earthquake map. Accessed November 20, 2013.
Yeats, R. Active Faults of the World. (2012). Cambridge University Press. Accessed November 20, 2013.